When a sentence is made up of two simple sentences joined by a conjunction (e.g., and, or, but), it is possible to precede the conjunction with a semicolon if either of the simple sentences contains a comma(s). This is quite an outdated practice, but it still can be used to improve readability.|
For example: Mark, 17, joined the Army; and Paul, Mark's younger brother, joined the Marines.
Semicolon before AndAs covered in the lesson Commas before Conjunctions, when a conjunction (e.g., and, but, or) merges two simple sentences into one, it should be preceded by a comma. However, if one (or both) of the sentences already contains a comma (or commas), then a semicolon can be used instead of a comma to outrank the commas within the simple sentences.
Using a semicolon in this way is quite an outdated practice. However, you can still use a semicolon if you think it makes things clearer for your readers than using a comma (which would the usual practice these days).
As these two sentences both contain commas, they can merged into one sentence using a conjunction (in this case but) preceded by a semicolon.
(Remember, it would be far more common to use a comma and not a semicolon.)
A sentence made up of two simple sentences is called a compound sentence. The conjunction that joins the two halves of a compound sentence should be preceded by a comma or possibly a semicolon.